Many industrial processes spray compositions that contain viscous or solid polymeric components, such as coatings, adhesives, release agents, additives, gel coats, lubricants, and agricultural materials. To spray such materials, it has been common practice to use relatively large amounts of organic solvents. The solvents perform a variety of functions, such as to dissolve the polymers; to reduce viscosity for spraying; to provide a carrier medium for dispersions; and to give proper flow when the composition is sprayed onto a substrate, such as coalescence and leveling to form a smooth coherent coating film. However, the solvents released by the spray operation are a major source of air pollution.
There are several patents which disclose new spray technology that can markedly reduce organic solvent emissions, by using environmentally acceptable supercritical fluids or subcritical compressed fluids, such as carbon dioxide, to replace the solvent fraction in solvent-borne Compositions that is needed to obtain low spray viscosity: U.S. Pat. Nos. 4,923,720 and 5,108,799 disclose methods for using supercritical fluids for the spray application of coatings. U.S. Pat. No. 5,106,650 discloses methods for using supercritical carbon dioxide for the electrostatic spray application of coatings. U.S. Pat. No. 5,009,367 discloses methods for using supercritical fluids for obtaining wider airless sprays. U.S. Pat. No. 5,057,342 discloses methods for using supercritical fluids for obtaining feathered airless sprays. U.S. Pat. No. 4,882,107 discloses methods for using supercritical fluids to apply mold release agents, such as in the production of polyurethane foam. U.S. Pat. No. 5,066,522 discloses methods for using supercritical fluids to apply adhesive coatings.
Smith, in U.S. Pat. No. 4,582,731, issued Apr. 15, 1986; U.S. Pat. No. 4,734,227, issued Mar. 29, 1988; and U.S. Pat. 4,734,451, issued Mar. 29, 1988; discloses methods for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in supercritical fluid solvents, which may contain organic solvents. The concentration of said solutes are described as being quite dilute; on the order of 0.1 percent by weight. In conventional spray applications, the solute concentration is normally 50 times or more greater than this level.
The molecular sprays disclosed in the Smith patents are defined as a spray "of individual molecules (atoms) or very small cluster of solute" which are in the order of about 30 Angstroms in diameter. These "droplets" are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional methods that Smith refers to as "liquid spray" applications.
The conventional atomization mechanism of airless sprays is well known and is discussed and illustrated by Dombroski, N., and Johns, W. R., Chemical Engineering Science 18: 203, 1963. The coating exits the orifice as a liquid film that becomes unstable from shear induced by its high velocity relative to the surrounding air. Waves grow in the liquid film, become unstable, and break up into liquid filaments that likewise become unstable and break up into droplets. Atomization occurs because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. However, viscous dissipation markedly reduces atomization energy, so relatively coarse atomization typically results. Liquid-film sprays are angular in shape and have a fan width that is about the fan width rating of the spray tip. They characteristically form a "tailing" or "fishtail" spray pattern, wherein coating material is distributed unevenly in the spray. Surface tension often gathers more liquid at the edges of the spray fan than in the center, which can produce coarsely atomized jets of coating that sometimes separate from the spray. As used herein, the phrases "liquid-film atomization" and "liquid-film spray" are understood to mean a spray, spray fan, or spray pattern in which atomization occurs by this conventional mechanism.
As disclosed in the aforementioned related patents, supercritical fluids or subcritical compressed fluids such as carbon dioxide are not only effective viscosity reducers, they can produce a new airless spray atomization mechanism, which can produce finer droplet size than by conventional airless spray methods and a feathered spray needed to apply high quality coatings. Without wishing to be bound by theory, the new type of atomization is believed to be produced by the dissolved carbon dioxide suddenly becoming exceedingly supersaturated as the spray mixture enters the spray orifice and experiences a sudden and large drop in pressure. This creates a very large driving force for gasification of the carbon dioxide, which overwhelms the cohesion, surface tension, and viscosity forces that oppose atomization and normally bind the fluid flow together.
A different atomization mechanism is evident because atomization appears to occur right at the spray orifice instead of away from it. Atomization is believed to be due not to break-up of a liquid film from shear with the surrounding air but, instead, to the force of the expanding carbon dioxide gas. Therefore, no liquid film is visible coming out of the nozzle. Furthermore, because the spray is no longer bound by cohesion and surface tension forces, it leaves the nozzle at a much wider angle than normal airless sprays and produces a "feathered" spray with tapered edges like an air spray. This produces a rounded, parabolic-shaped spray fan instead of the sharp angular fans typical of conventional airless sprays. The spray also typically has a much wider fan width than conventional airless sprays produced by the same spray tip. As used herein, the phrases "decompressive atomization" and "decompressive spray" are understood to mean to a spray, spray fan, or spray pattern that has the preceding characteristics.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the polymeric coating composition. This practical upper limit is generally recognizable when the admixture containing coating composition and supercritical fluid breaks down from one phase into two fluid phases. To better understand this phenomenon, reference is made to the phase diagram in FIG. 1, wherein the supercritical fluid is carbon dioxide. The vertices of the triangular diagram represent the pure components of a coating formulation admixed with carbon dioxide, which for the purpose of this discussion contains no water. Vertex A is solvent, vertex B is carbon dioxide, and vertex C represents a polymeric material. In this diagram, the polymer and the solvent are completely miscible in all proportions and the carbon dioxide and the solvent are likewise completely miscible in all portions, but the carbon dioxide and the polymer are not miscible in any portion, because the carbon dioxide is a non-solvent for the polymer. The curved line BFC represents the phase boundary between one phase and two phases. The point D represents a possible coating composition to which carbon dioxide has not been added. The point E represents a possible composition of a coating formulation admixture after addition of supercritical carbon dioxide. The added supercritical carbon dioxide is fully dissolved and has reduced the viscosity of the viscous coating composition to a range where it can be readily atomized by passing it through an orifice such as in an airless spray gun. After atomization, the carbon dioxide vaporizes, leaving substantially the composition of the original viscous coating composition. Upon contacting the substrate, the liquid mixture of polymer and solvent coalesces to produce a smooth coating film on the substrate. The film forming pathway is illustrated in FIG. 1 by the line segments EE'D (atomization and decompression) and DC (coalescence and film formation).
Although the supercritical fluid spray methods have been successful, one difficult problem that is created is that the reformulated polymeric composition, which is called a concentrate, has increasingly higher viscosity as higher levels of solvent are removed to further reduce solvent emissions. Concentrate viscosities typically increase from a conventional viscosity of about 100 centipoise to about 800 to 5000 centipoise or higher as more solvent is removed. Therefore, obtaining fine atomization becomes increasingly more difficult. This limits the amount of solvent that can be removed and hence the solids level that can be used in the concentrate. The poorer atomization gives poorer spray application quality such as poorer coatings. Therefore a need clearly exists for methods by which atomization can be enhanced when using supercritical fluids or subcritical compressed fluids to spray polymeric compositions in order to reach higher solids levels and to obtain finer atomization to obtain improved spray application quality.